The problem of stress corrosion cracking in the heat affected zones of welds of stainless steel piping in nuclear reactors has long been recognized. This problem has been summarized in U.S. Pat. No. 4,049,186 which was issued on Sept. 20, 1977. This patent outlines a number of existing methods of solving this problem. Among the methods or "fixes" are solution annealing after welding to desensitize the heat affected zone, controlling the temperature of the inside surface or inside diameter of the pipe during welding, and using inlays of weld material before the welding. These methods cannot be applied to existing installed pipe joints without cutting into the pipe and interrupting the plant operation.
The above patent demonstrates a method for reducing stress corrosion cracking in a weld by applying an overlay weld. This overlay weld extends across the pipe joint and bridges on each side of the pipe joint beyond the actual extremity of the primary weld heat affected zone at the inner surface of the pipe. This method applies additional weld material above the weld joint to increase the pipe wall thickness at the weld joint, thereby reducing the applied stress. It was a well held theory when this patent issued that if the applied stress at the joint could be reduced below a certain level that this would inhibit stress corrosion cracking. The addition of welding material increased the cross-sectional area and as the applied stress is directly proportional to the applied load, and inversely proportional to the cross-sectional area, the stress is decreased.
In this reference the welding material was deposited over the pipe joint in two manners. The first manner included the formation of a ring extending about the circumference of the pipe and covering the primary weld. The ring was composed of a plurality of weld beads which were laid down axially along the pipe. Another method disclosed included the formation of a series of separate weld segment ribs extending axially along the pipe at the weld joint and spaced apart radially around the circumference of the pipe.
More recently, a new theory has been put forth as to the cause of stress corrosion cracking. This theory, as presently accepted, holds that residual tensile stresses are built up at the pipe joint when the initial weld is made and that these welding residual stresses play at least as significant a role as applied stresses in causing stress corrosion cracking when the pipe is in use.
Accordingly, it is believed that stress corrosion cracking can be reduced or eliminated by removing this residual tensile stress or by inducing a compressive stress. Initial findings in this area were reported in an Electric Power Research Institute publication, RP449-2 entitled "Residual Stress Measurements on Type 304 Stainless Steel Welded Pipes" and was presented on Jan. 22, 1980. One of the solutions presented in this paper included the ring overlay previously described in the above patent. It is believed that the other solution including the use of separate ribs induces even greater residual stresses and thus would not be appropriate for use with the new theory.
It has been found, however, that the overlay weld method of the above reference does not provide the most efficient and effective manner of solving the stress corrosion cracking problem. The present invention is directed to solving this problem in a more effective manner.